1. Field of the Invention
The present invention relates to a back mirror for a vehicle capable of adjusting reflectance by having an electrochromic mirror (EC mirror) mounted on the vehicle, and relates, more particularly, to a technique for reducing power consumption of the EC mirror.
2. Description of the Prior Art
When a driver is driving a vehicle during nighttime, it often occurs that illumination light beams irradiated from the headlight of a vehicle following at the back of this vehicle are reflected by the back mirror (or an outer mirror or an inner mirror) and enter the driver""s visual field so that the driver is glared by the beams. In order to prevent this problem, there has been conventionally invented and put into practical application an EC mirror that is arranged by having an electrochromic element (EC element) mounted on a back mirror so that a DC voltage is applied to this EC element to change reflectance, thereby preventing a driver from being glared by the light irradiated from the vehicle at the back of the driver""s vehicle.
As has been widely known, when a DC voltage is applied to two input terminals in a forward direction, the surface of the EC mirror is colored to reduce the light reflectance so that the driver is prevented from being glared by the light reflected from the mirror. When a DC voltage is applied to the two input terminals in a backward direction or when the two input terminals are short-circuited, it is possible to return the light reflectance of the EC mirror to the original state.
Such an EC mirror has been conventionally known as disclosed, for example, in Japanese Utility Model Registration Publication No. 60540/94 (hereinafter referred to as a conventional example). FIG. 5 is an explanatory diagram showing a configuration of an EC mirror and its driving circuit described in this conventional example. As shown in FIG. 5, a DC power source 105 is connected to two input terminals 102 and 103 of an EC mirror 101 via a two-contact type and two-way switch 104.
When the switch 104 is operated to apply a voltage output from the DC power source 105 so as to set the input terminal 102 to a positive electrode and the input terminal 103 to a negative electrode, the EC mirror 101 is colored on its surface, which results in lowering of light reflectance. On the other hand, when the switch 104 is operated to apply a voltage output from the DC power source 105 so as to set the input terminal 102 to a negative electrode and the input terminal 103 to a positive electrode, the EC mirror 101 is discolored on its surface, which allows a light transmittance to be returned to the original state. Based on these operations, it is possible to change over between a glare-proof and a glare-non-proof.
According to the driving circuit of the above-described conventional EC mirror, however, it is necessary to keep the switch 104 depressed for a predetermined period of time in order to color or discolor the surface of the EC mirror 101, which is troublesome. In other words, the EC mirror 101 has a characteristic that the surface of the EC mirror 101 is gradually colored when a voltage is applied continuously to the input terminals 102 and 103 in a forward direction for a constant period of time, and that the EC mirror 101 is gradually discolored when a voltage is applied continuously to the input terminals 102 and 103 in a backward direction for a constant period of time. Therefore, the driver needs to keep depressing the switch.
It is an object of the present invention to provide a back mirror having an automatic glare-proof function capable of automatically switching between the coloring and discoloring of an EC mirror and capable of reducing power consumption of a DC power source.
According to one aspect of the present invention, there is provided a back mirror having an automatic glare-proof function, in which a DC voltage output from a battery E1 is normally supplied to a daytime/nighttime deciding circuit 3. Based on this arrangement, a first surrounding light sensor S1 makes a decision as to whether it is currently daytime or nighttime. When a decision has been made that the current time is daytime, an integrating circuit 4 operates to turn off an electronic switch FET1, thereby stopping a supply of DC voltage to a glare-proof/glare-non-proof deciding circuit 5 and a driving circuit 6. On the other hand, when a decision has been made that the current time is nighttime, the integrating circuit 4 operates to turn on the electronic switch FET1, thereby starting a supply of DC voltage to the glare-proof/glare-non-proof deciding circuit 5 and the driving circuit 6.
The integrating circuit 4 is set with a first time constant and a second time constant so that a time required for changing a decision from daytime to nighttime and a time required for changing a decision from nighttime to daytime can be set separately.
When a DC voltage has been applied to the glare-proof/glare-non-proof deciding circuit 5, a decision is made based on a backward light sensor S3 and a second surrounding light sensor S2 as to whether the EC mirror 2 is to be set to a glare-proof state or a glare-non-proof state. In other words, when the illuminance of the light from the back is large, the glare-proof/glare-non-proof deciding circuit 5 outputs a signal that makes the EC mirror 2 to the glare-proof state, and when the illuminance of the light from the back is small, the glare-proof/glare-non-proof deciding circuit 5 outputs a signal that makes the EC mirror 2 to the glare-non-proof state.
When the glare-proof/glare-non-proof deciding circuit 5 has output the signal that makes the EC mirror 2 to the glare-proof state, the driving circuit 6 applies a DC voltage to the EC mirror 2 so that the EC mirror 2 is set to a colored state, i.e. glare-proof state. On the other hand, when the glare-proof/glare-non-proof deciding circuit 5 has output the signal that makes the EC mirror 2 to the glare-non-proof state, the driving circuit 6 short-circuits between two input terminals Ta and Tb of the EC mirror 2 so that the EC mirror 2 in a colored state is switched to a discolored state, i.e. glare-non-proof state. In this way, it is possible to automatically change over between the glare-proof state and the glare-non-proof state.
Further, according to another aspect of the present invention, there is provided a back mirror having an automatic glare-prrof function, in which when the daytime/nighttime deciding circuit 3 has made a decision that the current time is nighttime, a DC voltage output A from the battery E1 is applied to the glare-proof/glare-non-proof deciding circuit 5 and the driving circuit 6. Therefore, it is possible to extremely reduce the power consumption of the battery E1, which elongates the cycle of replacing the battery E1.
In addition, since the glare-proof state and the glare-non-proof state of the EC mirror 2 can be automatically changed over between them, a driver does not need to operate to change over between the glare-proof state and the glare-non-proof state. As a result, a good operability as well as improvement of safety during the driving of the vehicle can be achieved.
Furthermore, according to the present invention, the daytime/nighttime deciding circuit 3 comprises a first Schmitt trigger circuit IC1-1, which makes it possible to prevent an occurrence of hunting without making a reaction to a slight change in the surrounding light. As a result, a secure decision can be made between the daytime and the nighttime.